Landscape context shapes bird responses to hedgerows in California’s Central Valley

Background/Question/Methods

Agricultural development in California's Central Valley has reduced habitat for native bird populations; however, anecdotal evidence suggests that planting linear strips of native vegetation, called hedgerows, between fields could benefit birds.  We surveyed birds at hedgerow and nearby control sites (unmanaged field margins) to estimate the effects of hedgerows on bird species richness and abundance.  Twenty hedgerow and 20 control sites were surveyed during each of two seasons, winter 2012-2013 and spring 2013, noting species composition and abundance.  In order to assess the effects of landscape context, we considered additional landscape-scale covariates in our analyses, incorporating data on adjacent field type, distance to urban, forest, and riparian habitats, and the heterogeneity of the surrounding landscape.  Data were analyzed using mixed and standard generalized linear models.  Results were then compared between models built with just a hedgerow covariate, and those that included other landscape features and their interaction with hedgerows.

Results/Conclusions

Hedgerows had a positive effect on species richness and abundance during both winter and spring surveys.   During the spring, significantly more taxa were restricted to hedgerows than control sites or were more abundant at hedgerow sites.  Landscape context, however, mattered when describing patterns in richness and abundance.  The effect of hedgerows on species richness decreased and became less significant when other landscape-level covariates were introduced into the model.    The opposite held for average abundance models.  Contrasting patterns in AIC scores lend support to this result, with AIC favoring the hedgerow-only model when richness was the response variable, and the full model with average abundance.  The two response variables therefore responded differentially to the influence of landscape-scale covariates, with species richness being more influenced by local factors and average abundance by the surrounding landscape. These results underscore the potential for landscape context to shape community-level responses to localized management actions.  Conservation planning, even when focused at smaller scales, must recognize the influence of landscape context and incorporate a regional perspective into the decision-making process to maximize the chances of success.